1. Field of the Invention
This invention pertains to methods and apparatus for preparing flat ribbon cables for termination by removing a portion of the outer cable insulation. More particularly, the present invention pertains to the notching of a flat ribbon cable to allow termination at a point remote from the cable ends.
2. Brief Description of the Prior Art
Flat ribbon cables are extensively used in many areas of electronic machine manufacture. For example, they are employed to interconnect various circuit boards and other modules within computer systems. Several techniques have been employed for terminating flat ribbon cables, such as preparing the cable ends with grinding wheels or stripping blades to bare the individual conductors of the cable, in preparation for a solder securement to a printed circuit board or the like.
However, it has often been found convenient to also terminate the flat ribbon cable at a point remote from the cable ends. This "daisy chain" technique is typically employed with mass termination electrical connectors having insulation displacing terminals.
Typically, flat ribbon cables are constructed to have a plurality of spaced-apart conductors, with a common dielectric insulating jacket surrounding the conductors and filling the spaces therebetween to form webs of insulation material. Conventional cable notching processes employ tool punches which remove only the insulating webs between adjacent conductors, leaving a resulting cylindrical sheath of conductor insulation. These processes produce cable portions which approximate the geometry of a discrete insulated wire having a circular outer periphery.
Economic considerations dictate that all notching of a given cable termination region be performed simultaneously. However, this creates undesired stresses in the cable, particularly with small gauge electrical conductors, in that a significant distorting force is applied to the cable conductors during the punching operation. Further, these punching operations have not been found to be satisfactory to produce a bare wire, if one is required in the notched area.
One example of a notching operation that can be used to produce bar conductors is described in U.S. Pat. No. 4,442,587 wherein a pair of opposed, rotatably driven grinding wheels abrade the outer cable insulation. In this operation, the insulation on the bottom of the cable is pushed in one direction, while the insulation on the top of the ribbon cable is pushed in the other direction, with the grinding wheels being moved transversely across the cable. It is possible, during the grinding operation, that the outer surface of the cable conductors would be damaged, a particularly undesireable risk when wires are topcoated with precious or corrosion-resistant metals.
Recently, objections have been raised against flat ribbon cables which radiate undesireable signal emissions during operation of electronic equipment. Various arrangements for shielding flat ribbon cables have been provided to reduce the level of signals emitted from a cable conductor, to electrically isolate it from other sources, and to protect the cable from sudden electrostatic discharge.
A particular type of shielded cable, commonly identified as a transmission cable, has carefully controlled electrical characteristics such as a matched impedance, and a low capacitance. Control over impedance and capacitance can be provided only with close manufacturing tolerances of the shielded cable. One example of a shielded cable having these desireable electrical properties is described in commonly owned U.S. patent application Ser. No. 032,360 filed Apr. 23, 1979, abandoned. The cable includes a plurality of spaced-apart insulated conductors and at least one bare conductor used as a drain wire. The insulated conductors and the bare drain conductor are embedded in a condutive polymer substrate, comprising an integral homogenous encapsulating medium which surrounds the conductors and the spaces therebetween. Surrounding the conductive polymer substrate is a conventional outside insulation jacket which is either laminated or extruded around the cable subassembly. The conductor insulation employed in the cable to surround the individual cable conductors can be a conventional polymeric insulation, such as polyvinyl chloride, or may be a thin varnish coating applied to the cable conductor.
Previous cable notching arrangements cannot be satisfactorily applied to the conductive polymer shield cable, due to the presence of the encapsulating conductive substrate which must be entirely removed from the prepared cable area to prevent shorting between adjacent terminals. The punching method described above would leave layers of conductive material on the top and bottom of each cable conductor, and the aforementioned grinding preparation techniques would leave conductive material between adjacent cable conductors. Further, the above-described grinding techniques could not be employed if the cable is to be terminated in a connector having insulation displacing terminals.
Even if the grinding techniques of removing cable insulation could otherwise be employed, the risk of damaging a varnish or other delicate wire coating would prohibit their use.